The present invention relates to automatic dynamic signal compression and expansion systems, particularly for use in high fidelity sound reproduction equipment.
It is known that the quality of the reproduction of audio frequency performances can be improved by increasing the signal to noise ratio by means of a compression-expansion system known as a compander system, in which the useful signals are compressed in amplitude before they enter a transmission path susceptible to interference, e.g. a line or a recording tape, and are expanded after transmission.
Design of the known systems is based on the goal of meeting, inter alia, the following requirements to the maximum extent possible: low cost, corresponding characteristics during compression and expansion, good reproducibility of the required values in instruments of, for example, a mass-fabricated series, and the expandability of the system concept with respect to the selection of the degree of expansion and compression. The known compression-expansion systems are either relatively expensive and produce good quality or they are less expensive, as required for instruments sold to the general public, and do not meet the above requirements to a satisfactory degree.
German Auslegeschrift [Published Patent Application] No. 24, 06, 258 and corresponding U.S. application Ser. No. 543,931, filed on Jan. 24th, 1975, and now U.S. Pat. No. 3,969,680, disclose a circuit for automatic dynamic compression or expansion in a useful signal path which includes means for varying the transmission factor. In this circuit, a path branches off from the output or input, respectively, of the useful signal path, a control voltage is derived from the useful signal path in a signal-responsive control voltage generator disposed in the branch path, and this control voltage is fed to the means for varying the transmission factor in the useful signal path. The control voltage is also fed to means for varying the transmission factor in the branch path in a manner to effect a counterregulation of the transmission factor in the branch path.
This counterregulation causes the control characteristics for compression and expansion which will be referred to hereinafter simply as characteristics, and which are represented by the dependence of the output signals U.sub.2 upon the input signals U.sub.1 as shown in FIG. 1, to become approximately linear on a logarithmic scale of the signal voltages, so that their reproducibility in the instruments of a mass-produced series becomes easier. Normal fluctuations in the properties of the individual components have no significant influence.
The characteristics attainable with the described circuit exhibit, as shown in FIG. 1, a first, linear part 1 with a slope corresponding to the constant gain of the system at low input levels and a second, likewise linear portion 2 with a slope which is less than that in the first portion 1 and which corresponds to a dynamic compression. When a certain control value has been reached, for example if the so-called rated or reference level has been reached in recording studio systems, the characteristic changes to a third linear portion 3 which again corresponds to the predetermined constant gain.